The Daily Show has done the joke that Bush may have invaded the wrong country, but in his defense there is only one letters difference between Iraq and Iran. It's an honest spelling mistake. On a more serious note check out Informed Comment for what exactly we can do about Iran's nukes:
The US can no longer mount a land campaign against Iran. It is bogged down in Iraq fighting a guerrilla war. The Iraqi Shiites would never put up with a US campaign against Shiite Iran. US stockpiles of smart weaponry are seriously depleted.
The US could not make up any slack from allies such as the UK. Tony Blair's parliamentary majority has been whittled down to only 66. About 40 Labour MPs routinely vote against his bills and intitiatives anyway. So his real majority may be razor-thin. He certainly could not convince this parliament to go to war in Iran, e.g.
So the military options are not apparent.
What are the covert options? It seems highly unlikely that the Iranian officer corps would cooperate in a coup. The clerics would be very difficult to overthrow in any popular movement as long as the police, intelligence services and army are loyal to the state and anti-American. Iranian nationalism is fierce and the population would support the government against a foreign invasion.
The Iraq War has really hurt our ability to deal with other threats. North Korea has nukes, Iran will have nukes and we have nothing. Iraq is not just bad on its own, its doing real damage to the safety of America in the World.